As a transmission for a vehicle, there are known; a type of transmission that carries out a shift by selecting from among a plurality of power transmission paths having predetermined speed ratios, and a transmission having a mechanism that is able to continuously change its speed ratio. A typical example of the former transmission is a stepped gear transmission, and a typical example of the latter transmission having the mechanism is a belt-type or toroidal-type continuously variable transmission. A gear transmission mechanism and a continuously variable transmission mechanism each are able to constitute a transmission alone; however, a combination of these mechanisms is able to set further various speed ratios and is also able to constitute a compact transmission.
As one example of that, Japanese Utility Model Application Publication No. 62-45455 (JP 62-45455 U) describes an automatic transmission in which a belt-type continuously variable transmission and a gear train are arranged in parallel with each other between an input shaft and an output shaft. The belt-type continuously variable transmission includes a primary pulley and a secondary pulley around which a belt is wound. The input shaft is coupled to the primary pulley via a gear. The secondary pulley is coupled to an intermediate shaft via a clutch. On the other hand, in the gear train, a gear formed on a drum of the clutch serves as a drive gear, the drum is selectively coupled to the input shaft, and the drive gear is in mesh with a forward gear connected to the intermediate shaft. A reverse gear is rotatably connected to the intermediate shaft. A switching sleeve is arranged between the reverse gear and the forward gear. The switching sleeve is an intermeshing engagement mechanism. When the switching sleeve moves in the axial direction to mesh with the forward gear, the switching sleeve couples the forward gear to the intermediate shaft. When the switching sleeve moves toward a side opposite to this to mesh with the reverse gear, the switching sleeve couples the reverse gear to the intermediate shaft. The reverse gear is coupled to a reverse counter gear via an idle gear. The reverse counter gear is connected to the output shaft.
The transmission described in JP 62-45455 U includes not only the belt-type continuously variable transmission but also the gear transmission mechanism, so the transmission carries out a shift by engaging or releasing the clutch or the switching sleeve. A shift for operating an engagement mechanism, such as a clutch, is general in a stepped transmission; however, when the engagement mechanism is operated, there occurs fluctuations in rotation speed and torque, so it may be required to execute control for a shift shock or improvement in durability. For example, an apparatus described in Japanese Patent Application Publication No. 2004-270891 (JP 2004-270891 A) is configured to execute control for reducing a rotation speed difference in engaging a synchronization mechanism in order to improve the durability of the synchronization mechanism in a stepped transmission. The configuration of the apparatus described in JP 2004-270891 A will be simply described. A pair of drive gears are rotatably connected to a sun gear shaft of a planetary gear train to which torque is transmitted from an engine via a plurality of clutches. A pair of driven gears respectively in mesh with the corresponding drive gears are connected to an output shaft. A dog clutch is provided between those drive gears, one of the drive gears is configured to be selectively coupled to the sun gear shaft by the dog clutch. Another pair of drive gears are connected to a carrier shaft integrated with a carrier of the planetary gear train. Another pair of driven gears respectively in mesh with the corresponding drive gears are rotatably connected to the output shaft. Another dog clutch is arranged between these driven gears. One of the driven gears is configured to be selectively coupled to the output shaft by the dog clutch. Thus, the transmission described in JP 2004-270891 A includes the four gear pairs as gear pairs that transmit torque to the output shaft, and is configured to select the gear pair that transmits torque with the above-described two dog clutches. In the case of a shift that changes a rotating element to which torque is transmitted from the engine among rotating elements of the planetary gear train and that changes the engaged/released states of the dog clutches, a so-called tie-up state is temporarily established. Thus, a rotation speed difference in the synchronization mechanism that should be engaged is reduced. In the tie-up state, the dog clutch that should be engaged after the shift is set to the released state and then torque is transmitted from the engine to two rotating elements of the planetary gear train in this state.
Incidentally, the switching sleeve described in JP 62-45455 U and the dog clutches described in JP 2004-270891 A each are a mechanism that transmits torque by meshing teeth with each other. Therefore, a normal meshing state is a state where the drive-side teeth are shifted by a half pitch from the driven-side teeth. In other words, when the teeth are shifted by a half pitch in this way in a non-engaged state (released state), those teeth are allowed to mesh with each other when the teeth are brought close to each other. On the contrary, when there is no shift between the phases of the teeth in the non-engaged state (released state), if the teeth are brought close to each other in order to set the switching sleeve or one of the dog clutches to the engaged state, the teeth collide with each other, with the result that the teeth cannot be meshed with each other.
Thus, for example, in the transmission described in JP 62-45455 U, if a speed ratio of the gear train is set so as to be higher than a maximum speed ratio of the belt-type continuously variable transmission and torque is configured to be transmitted to the output shaft by the gear train at the time when the vehicle starts moving, when the phase of the switching sleeve coincides with the phase of the teeth of the forward gear or reverse gear in a vehicle stopped state, the teeth of the switching sleeve contact the gear-side teeth, with the result that the forward gear or the reverse gear is not coupled to the intermediate shaft. If the gear train is coupled to the input shaft while the teeth contact with each other, the forward gear rotates to cause a shift between the phases of the teeth, so it is possible to mesh the teeth with each other. However, torque of the drive wheel steeply increases in the vehicle stopped state, so there is a possibility that a shock occurs.
In the transmission described in JP 2004-270891 A, a rotation speed difference in the dog clutch placed in a standby state is eliminated in a short time, so the rotation speed difference that is absorbed by the synchronization mechanism decreases, with the result that it is possible to improve the durability. However, because the synchronization mechanism is a mechanism that exercises the function of synchronizing the rotation speeds of members to be coupled while at least one of the members is rotating, the synchronization mechanism does not exercise the synchronization function for meshing when rotation of both members is stopped. The apparatus described in JP 2004-270891 A is an apparatus for reducing the rotation speed difference in the synchronization mechanism in the case of carrying out a shift while the vehicle is traveling, and cannot be directly applied to control for reliably and smoothly meshing the teeth of the engagement mechanism in a state where the engagement mechanism is not rotating, such as a state where the vehicle is stopped.